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Securities (Year End Securities with Unrealized Losses Segregated by Length of Impairment) (Detail) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Schedule of Available-for-Sale Securities [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months $ 2,126,451  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 33,381  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 11,658,462  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 1,529,997  
Available for Sale, Total Estimated Fair Value 13,784,913  
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax 1,563,378 $ 1,401,795
US Treasury [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 0  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 3,442,320  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 249,895  
Available for Sale, Total Estimated Fair Value 3,442,320  
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax 249,895 285,305
Residential Mortgage Backed Securities [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 1,289,852  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 26,420  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 4,725,498  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 1,002,734  
Available for Sale, Total Estimated Fair Value 6,015,350  
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax 1,029,154 876,338
States and Political Subdivisions [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 836,599  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 6,961  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 3,490,644  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 277,368  
Available for Sale, Total Estimated Fair Value 4,327,243  
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax $ 284,329 $ 240,152